The present invention relates to a method of determining the pole arc ratio of an interior permanent magnet rotary motor and the interior permanent magnet rotary motor.
Generally, there is known a permanent magnet rotary motor constituted from an armature that creates a shifting magnetic field and a magnetic field system constituted from permanent magnets that act with the shifting magnetic field to generate an electromagnetic force. However, in the motor that uses the permanent magnets as described above, pulsating torque (cogging torque) and a thrust are produced at the time of no-load operation. Such cogging torque hinders smooth rotation or reciprocation of the motor, thus causing vibration and speed variation of the motor. As an approach to reducing the cogging torque, there have been hitherto proposed formation of a slot with an oblique groove, hereinafter referred to as a skew, in a stator or a rotor, or use of a permanent magnet having the shape of a sector with the center of an inner circular arc thereof not aligned with the center of an outer circular arc thereof, hereinafter referred to as the permanent magnet of an eccentric type. However, formation of the skew may reduce the torque or reduce the productivity of the motor. Further, use of the permanent magnet of the eccentric type cannot increase the magnetic flux density of the motor, so that higher torque density cannot be achieved.
Then, in the interior permanent magnet rotary motor disclosed in Japanese Patent Application Laid-Open Publication No. 299199/1999 (Patent Document 1), in which permanent magnets are embedded within a rotor core to form permanent magnet magnetic pole sections, setting of the following condition was proposed:θ≈n×τs+16/Pin which an angle formed between two line segments each of which connects the center of the rotor and one of the corners in the peripheral direction of each permanent magnet magnetic section located on the side of the surface of the rotor core, or a permanent magnet angle is indicated by θ. Then, a slot pitch is indicated by τs, and the number of the permanent magnet magnetic pole sections is indicated by P, and n is a natural number.
However, in the conventional setting, though the torque caused by the permanent magnets, referred to as the magnet torque, can be maintained to be high, the width dimension of each permanent magnet that extends in a direction orthogonal to the diameter direction of the rotor core will be increased. Thus, the amount of the permanent magnets used will be increased, so that the cost of material will be increased.
An object of the present invention is therefore to provide a method of determining the pole arc ratio of an interior permanent magnet rotary motor and the interior permanent magnet rotary motor that can reduce cogging torque and also can reduce the amount of permanent magnets used, without reducing torque.